The present invention relates to an image-forming optical system using an optical low-pass filter and also relates to an apparatus using the image-forming optical system. More particularly, the present invention relates to an image-forming optical system using an optical low-pass filter, which is suitable for use in digital still cameras, video cameras, endoscopes and so forth, which use a solid-state image pickup device, e.g. a CCD. The present invention also relates to an apparatus using such an image-forming optical system.
In recent years, a great variety of small-sized image pickup apparatus using a solid-state image pickup device, e.g. a CCD, have been commercially manufactured as represented by digital still cameras. Solid-state image pickup devices used in these apparatus are arranged to obtain desired image information concerning a subject by spatial sampling.
In general, solid-state image pickup devices have a discrete pixel structure. Therefore, if the subject contains a spatial frequency component higher than the sampling frequency, the solid-state image pickup device may generate a false signal, which is not originated from the subject, e.g. moire fringes, and also cause a beat phenomenon in which a fine fringe pattern becomes a thick fringe pattern having undulations in density. In addition, a false color may be produced. To minimize the influence of these undesired phenomena in an image pickup apparatus that uses a solid-state image pickup device, e.g. a CCD, an optical low-pass filter is placed in the optical system to limit high spatial frequency components of the subject that enter the solid-state image pickup device.
Generally known conventional optical low-pass filters are a low-pass filter that utilizes the birefringence of a uniaxial crystal such as quartz crystal, and a phase type low-pass filter that utilizes the diffraction phenomenon.
In the case of a quartz crystal filter, rays are separated by utilizing birefringence. Therefore, the amount of separation of rays depends on the thickness of the filter. Meanwhile, a necessary amount of ray separation is determined by the pixel pitch. Therefore, even if the size of the solid-state image pickup device reduces, the size of the quartz crystal filter does not change unless the pixel pitch changes. For this reason, it is difficult to apply the quartz crystal filter to optical apparatus that are required to be compact in size. Moreover, the quartz crystal filter is costly and hence makes it difficult to achieve a low-cost optical apparatus.
On the other hand, the phase type low-pass filter can be produced simply by forming a diffraction grating or the like on a substrate. In addition, the amount of ray separation depends on the pitch of the diffraction grating. Therefore, it is possible to achieve a thin low-pass filter. Furthermore, if a plastic or other similar material is used as a substrate, it is also possible to reduce the cost.
In general, the quartz crystal filter is placed between a solid-state image pickup device and an optical system that is telecentric on the image side, in order to obtain a necessary amount of separation of rays on the image plane. On the other hand, it is desirable for the phase type low-pass filter to be placed in the vicinity of the pupil of the optical system from the viewpoint of obtaining the low-pass filter effect substantially uniformly for the axial and extra-axial rays. Thus, the phase type low-pass filter can be placed inside the optical system, and it is possible to reduce the size of the optical system. There have heretofore been made a large number of propositions concerning the structure of optical low-pass filters, particularly phase type low-pass filters. However, there has been proposed no effective arrangement for achieving a compact optical system using such a low-pass filter structure.
There have also been proposed a large number of techniques wherein a reflecting surface is used for the purpose of reducing the thickness and size of an optical system using an optical low-pass filter [for example, see Japanese Patent Application Unexamined Publication (KOKAI) Nos. 8-292368 and 8-292371]. However, there is no prior art in which a thin and compact optical system is achieved by positively considering the arrangement and placement of an optical low-pass filter.